Transformer Laminated Cores: A Complete Guide to Materials, Types, and Efficiency

Every transformer core faces the same challenge: convert electrical energy into magnetic flux without producing heat. The solution? Laminated cores. By stacking thousands of thin, insulated silicon steel sheets instead of using a solid block of iron, engineers reduce eddy current losses by over 95%, extend equipment lifespan by decades, and control noise levels. Here’s everything you need to know about lamination mechanics, materials, and the trade-offs between core types.

A Brief Introduction to Laminated Cores

What are laminated cores?

The magnetic transformer core consists of hundreds of individual ultra-thin electrical steel sheets stacked together instead of a single block of metal. Each individual is usually 0.23 mm to 0.35 mm thick, and is paper-thin in terms of electrical insulation.

This multi-layer structure will simultaneously help to fulfill two functions:

· Give a High-Permeability Path: The steel lets the magnetic flux to go easily through the core.

· Block Electrical Conductivity: The insulating layers physically block currents from flowing in loops, making the unwanted flow of electrical currents between sheets very difficult.

Solid Cores vs Laminated Cores: What’s the difference?

The Science Behind It: How Laminated Cores Reduces Eddy Currents

What Are Eddy Currents in Transformers?

A current which circulates in a metallic conductor due to a changing magnetic field is called an eddy current, or Foucault’s current.

The magnetic flux in an AC transformer is continually changing. Faraday’s Law of Electromagnetic Induction says that this change in flux creates a voltage in the secondary winding, and within the core as well. Iron and steel are conductive materials and these electric potentials generate closed loops of currents swirling around within the iron or steel solids, just like whirlpools in water.

| Pro Insight: Engineers usually only focus on core lamination. In practice, however, 3D FEA simulations, such as 17 MVA / 35 kV class transformers, show that leakage magnetic fields can penetrate the core, generating high secondary eddy currents in structural steel, such as oil tank walls and clamping frames. Local flux densities as high as 0.083 T can be developed by unshielded structural eddy currents, which lead to high hot spots that cause deterioration of the transformer’s oil.

The Mechanism: How Thin Sheets & Insulation Break Current Loops

Transformer designers don’t change the magnetic characteristics of the steel, but instead they physically break the electrical path of the steel to prevent the eddy currents.

· The Layered Architecture: The core is made of thousands of super thin steel sheets, stacked side by side and parallel to the magnetic flux.

· Insulation Barrier: A very thin layer or coating, of a very high electrical resistance, is applied in each case to provide electrical insulating function, such as varnish or inorganic oxide coating.

· How It Works: Magnetic flux will freely pass from one sheet to the next, but the electrical resistance of the coating on the sheets will prevent the current from “jumping side to side”.

The Core Logic: The concept is to distribute a single large amplitude eddy current loop into thousands, each being isolated, by lamination. The losses of eddy current are proportional to the square of width of the path of the current, and the total eddy loss is very small due to these small loops.

How Laminated Cores Thickness Affects Core Efficiency

The classical eddy current loss formula P_e = k × f ² × B²_m × t ², shows that the power loss is directly proportional to the square of the lamination thickness (t). This mathematical relationship basically proves that the thickness control is very important in modern transformer production: for example, whether to use 0.23 mm, 0.27 mm or 0.35 mm sheet.

Why Silicon Steel Is Used in Laminated Cores?

What Are Silicon Steel Laminations?

Silicon steel, or electrical steel is a type of iron-silicon alloy that has been specially developed to have very high magnetic and electrical resistance.

Metallurgists add a specific quantity of silicon (which is typically 2%–4.5%) into the iron to get a material with a particular property of electromagnetic induction. Silicon steel laminations are typically used for the industry standard cores of transformers, the stators of electric motors and the inductors.

| Pro Insight: In addition to core loss, which is the most talked about noise source, magnetostriction, or “breathing” of steel in a magnetic field is the source of low frequency noise and structural vibration in transformers. Over-tight clamping or poor core assembly can cause compressive stress, which can increase the magnetostriction by almost 4 times, resulting in a quiet transformer becoming a noisy transformer.

The Role of Silicon in Laminated Cores

· Increases Electrical Resistivity: Pure iron conducts electricity too well, which is bad here as it allows eddy currents to flow freely and dissipate energy. The electrical resistance increases four to five times when silicon is added. The eddy currents can’t flow as strongly when there is more resistance. What is the best thing about it? It does this while not decreasing the magnetic flux carrying capacity of the steel.

· Reduces Hysteresis Loss: The small magnetic domains in the core have to change direction with every change in the direction of the AC voltage, which changes 50 or 60 times a second. The energy is lost as heat due to internal friction resulting from that flipping. These domains can be flopped with less force when silicon is added, and the hysteresis loop is made smaller, making the iron “softer” magnetically.

Key Manufacturing Process: From Raw Alloy to Finished Core

Cold Rolling: Cold rolling means being rolled into accurate thickness, such as 0.23mm-0.35mm under the effect of cold, the process causes the steel to be mechanically stressed and the crystalline grains are aligned.

Annealing: Thinning into thin laminations by stamping and shearing creates a great deal of mechanical stress that will destroy the alignment of the magnetic domains. The purpose of the high temperature stress-relief annealing is to restore the core’s magnetic permeability and reduce core losses.

Insulation Coating (Color & Classification): After the rolling and annealing stages, the sheet is coated on both sides with a microscopic inorganic or organic coating (C-3, C-5, phosphate coating etc.).

What the Color Tells You: The Commercial silicon steel laminations have different surface colors, depending on the type of coating and the method of processing:

| Real Cases in Point: In our shop, we would construct an S11-50000/110 core which is composed of 14,500 sheets of steel stacked 2,900 layers high; each layer is 28 tons of steel. It took 6 of our guys 16 hours to stack everything manually.

Types of Laminated Cores

Classified by Shape:

Classified by Material:

How Laminated Cores Improves Efficiency & Lifespan

· Reduce Energy Loss
Thinner laminations cut eddy current losses by 25–30%, saving 8,000–10,000 kWh/year (up to $ 30,000 over 30 years for a large core). The first year or more of high-grade steel premiums is offset by the savings within 3-4 years.

· Reduce Temperature Rise
The smaller the eddy current loops, the less internal core heat suppression. Each 8-10°C drop in temperature doubles the life of the insulation, so for every degree cooler the cores will be, the thermal degradation is slowed and the lifespan is extended for decades.

· Reduce Magnetostriction & Acoustic Noise
A new advanced step-lap lamination technique that controls clamping stress, reduces magnetostriction up to 90%. This cuts down on the overall noise level 2-5 dB and ensures complete noise compliance for installations where noise is a concern such as hospitals and residential areas.

Conclusion

Laminations eliminate the energy loss due to eddy currents. Magnetic path passes through the grain-oriented steel and is not blocked by the insulation and thin sheets which make up the loops. When successful, the core will not only operate quieter, but will consume less energy and provide longevity of decades.

Searching for high-efficiency, ultra-low noise transformer solutions? At OMEK, we design and produce custom laminated cores and complete distribution transformers to international grid standards. Contact us now!